1. Field of the Invention
The present invention relates to a modular scanning unit of a position measuring system for the photoelectric scanning of a scale at a fixed distance, having a support on which a holder can be adjusted.
2. Discussion of Related Art
A modular scanning unit in accordance with the species for the photoelectric scanning of a scale is described in U.S. Pat. No. 4,593,194, the entire contents of which are incorporated herein by reference. The modular scanning unit includes a holder, on which a light source, a lens, a scanning plate, as well as a detector array are fastened and aligned with each other. Such a holder can be produced relatively simply, and the structural elements attached to it can be fastened in an automated manner.
For the correct scanning of the scale it is necessary that the scanning distance of the scanning plate with respect to the scale is exactly set as a function of the grating parameters. It is furthermore necessary that the angular position of the scanning grating of the scanning plate is set in accordance with the requirements to match the graduations of the scale grating.
In accordance with U.S. Pat. No. 4,593,194, the holder is provided with a tube-shaped extension piece for this purpose, which is pushed onto a cylindrical trunnion as the support. The holder can be displaced on the trunnion, by which the scanning distance can be set, and the holder is furthermore rotatable around the trunnion in a horizontal plane, by which the setting of the moiré angle is made possible. After it has been set, the holder is fixed in place on the trunnion by clamping.
With this embodiment it is disadvantageous that the trunnion only permits a rotation in the horizontal plane and that moreover the extension piece increases the structural size of the modular scanning unit.
A modular scanning unit of a linear measuring device is represented in FIGS. 13A and 13C of EP 0 548 848 A, wherein a structural group including a light source, a lens, as well as a grating in a recess of a support, is freely adjustable with respect to further structural elements of the modular scanning unit. After an adjustment has been made, the modular group is clamped to the support by screws and is additionally secured by an adhesive.
The positive fixation by screws within the recess is elaborate and as a rule results in the loss of the adjustment. Furthermore, the gluing gaps for the adhesive are relatively large, which has negative effects on the sturdiness and long range stability.